1. Field of the Invention
The present invention refers to an enzyme electrode, the oxygen supply of which is satisfied through electrolysis.
2. Description of the Prior Art
An enzyme electrode is an electrode for quantitative chemical analysis. It consists of an electrochemical sensor (base sensor) the sensitive surface of which is coated with an immobilized enzyme. Usually, the enzyme is separated from the surrounding sample by means of a semi-permeable membrane which permits small molecules to pass but prevents leakage of the enzyme out from the electrode and protects the enzyme from a microbial attack of the sample.
The enzyme is chosen so as to react with the substance to be analyzed. This substance thus forms a substrate for the enzyme. The electrochemical sensor is chosen so as to measure the concentration or activity of any of the reagents of the enzyme reaction. When such an enzyme electrode is introduced into a solution containing the substrate of the enzyme, the substrate molecule diffuses into the enzyme layer and reacts with the enzyme. The reaction products diffuse out from the enzyme layer. The base sensor measures the concentration of one of the reagents in the enzyme layer. After a while an equilibrium is formed where the concentration of the reagents is constant at the surface of the base sensor. If the enzyme activity is sufficiently high as compared to the substrate concentrations of the enzyme layer, the concentration of the reagents will be directly proportional to the substrate concentration of the sample. The base sensor thus indirectly measures the concentration of the substrate of the enzyme in the sample. A more detailed description of the principle of the enzyme electrode and a list of enzyme electrodes known per se are given in Enfors, S-O. and Molin, N., Process Biochemistry, Vol. 13, pp. 9-11 and 24 (1978).
During the short period of existence of enzyme electrodes (about 10 years), big efforts have been made to design electrodes for analysis of glucose and other saccharides. Several glucose electrodes are described in the literature.
Their function is usually based on the following reaction: ##STR1##
As H.sub.2 O.sub.2 could have an inhibiting effect on the glucose oxidase one could possibly coimmobilize catalase with the glucose oxidase whereby the following reaction is added: ##STR2##
This reaction has the effect that half of the oxygen consumed in reaction (I) is recycled.
As a base sensor for the glucose electrode three different electrodes have been used: an oxygen electrode, a hydrogen peroxide electrode (both these being polarographic electrodes) and a pH electrode. If an oxygen electrode or a pH electrode are used, the glucose electrode could be based merely on reaction (I) or on the reactions (I)+(II). The hydrogen-peroxide electrode as a base sensor will however, require that only reaction (I) takes place, i.e. catalase is not present in the glucose electrode. The pH-based glucose electrode has the disadvantage that the response characteristic varies and is determined by the pH dependence of the buffering capacity of the sample, which results in that the electrode cannot be calibrated in advance for analysis of samples having unknown buffering properties. The sensitivity to glucose and the measuring range of the electrode is then primarily determined by the buffering capacity of the sample. This disadvantage is avoided by choosing one of the other base sensors (oxygen or hydrogenperoxide electrodes), whereby the response of the glucose electrode will be directly proportional to the glucose concentration. The measuring range of these electrodes is determined by the relation of the permeability for glucose and oxygen of the membrane in that a high ratio between the glucose permeability and the oxygen permeability gives an increased sensitivity whereas a small ratio gives a decreased sensitivity but a wider measuring range. At high rates of glucose the total enzyme activity of the electrode could also limit the measuring range as the function of the enzyme electrode requires that the enzymatic reaction is limited by the substrate.
In practice it has proved that the measuring range of all glucose electrodes hitherto used has had an upper limit of 1-2 glucose/l as the oxygen transfer rate to the enzyme will not permit a faster enzyme reaction. If the sample is not saturated with respect to oxygen which is often the case for instance in fermentation media, the measuring range will, because of reduced oxygen transfer rate, be further decreased. It is then not possible to extend the measuring range upwards by reducing the permeability of the membrane by using a thicker membrane as this gives rise to a corresponding relative reduction of the oxygen transfer rate to the enzyme.